1. Field of the Invention
The present invention relates generally to integrated circuit chips.
2. Description of the Related Art
Modern integrated circuit chips generally include large numbers of circuit elements such as transistors, capacitors and resistors that are arranged to provide numerous electronic systems. Typically, these circuit elements are physically constructed in a circuit substrate that is formed of a semiconductor such as silicon. To insure proper circuit operation, the chip structure must include some form of circuit isolation so that the various circuit elements properly interact with each other.
An exemplary form of isolation is constructed by the following method (which assumes use of a silicon semiconductor):                a) a substrate is formed of P-type silicon;        b) N-type silicon diffusion islands are formed in the P-type silicon substrate; and        c) circuit elements are constructed into the N-type islands.For example, an N-type island may be a well into which is formed a P-type metal-oxide-semiconductor (MOS) transistor. Alternatively, an N-type island may form the collector of an NPN bipolar junction transistor.        
To insure proper circuit operation, the N-type islands are then generally biased with a higher voltage than is the P-type silicon substrate so that the associated PN junctions are reversed biased. This generally means that the circuit substrate is biased negatively with respect to a circuit ground. With the N-type islands turned off (i.e., reverse biased) with respect to the silicon substrate, circuit elements are now properly isolated to insure intended circuit operation. Failure to achieve this isolation risks circuit malfunction and possibly destructive latch-up.